1. Field of the Invention
This invention relates to a bone assessment apparatus, and more particularly to an apparatus which uses ultrasonic waves to assess bone.
2. Description of the Prior Art
Many types of apparatus have so far been proposed to assess bone by using ultrasonic waves (ultrasound). For example, in a bone assessment apparatus disclosed by U.S. Pat. No. 3,847,141, the heel of the foot is enclosed between a pair of ultrasonic transducers, and the bone is assessed by passing ultrasonic waves through the heel.
In this conventional apparatus, soft rubber pads are provided on the front of each transducer. When there is an air layer having a considerably different acoustic impedance in the propagation path of the ultrasonic waves, the ultrasonic waves are attenuated and reflected by the air layer. The above rubber pads solve this problem.
Another type of bone assessment apparatus is disclosed by Tokkai (Japanese Patent Laid-Open) Hei 6-22960 (U.S. Pat. No. 5,348,009) and Tokkai Hei 6-254099. In this type of bone assessment apparatus, a pair of ultrasonic transducers are disposed a fixed distance apart in a tank filled with a coupling liquid, however it has been pointed out that handling of the tank is difficult.
Another type of bone assessment apparatus which does not use a tank is disclosed in Tokkai Hei 7-204205. In this conventional apparatus, each one of a pair of transducer assemblies equipped with ultrasonic transducers is disposed on either side of a body part, and a fluid bag containing a coupling liquid is disposed in front of the ultrasonic transducers. This fluid bag is relatively large. The surface of the bag in contact with the body part is rectangular, and it is slightly inflated on the outside (side in contact with the body part). The bag is capable of overall deformation, however the area of contact with the body part does not change even when the bag deforms. According to this conventional apparatus, the bag and coupling liquid are always interposed between the transducers and surface of the body part.
A similar type of bone assessment apparatus is disclosed in Tokkai Hei 7-303643. According to this apparatus, a pressure sensor which detects the pressure of the transducer assembly on the body part is provided inside the fluid bag. In this way, ultrasonic waves can be sent and received while maintaining a constant pressure.
However in all of the above bone assessment apparatuses, it was impossible to vary the cross-sectional area of the ultrasonic beam emitted toward the body part (i.e. the spot area of the ultrasonic beam on the bone surface). In other words, in the prior art, the width of the ultrasonic beam was not controlled.
The bone assessment apparatus commonly found today is used for diagnosing relatively elderly people who are more susceptible to bone diseases such as osteoporosis. Hence, the cross-sectional area of the ultrasonic beam was set to correspond with the bone of an adult (e.g. calcaneous or heel bone).
Notwithstanding this, in recent years, there is an increasing need to assess the bones of younger persons (children) to prevent and diagnose bone disease. Prevention and early treatment require early bone diagnosis.
However the conventional bone assessment apparatus was set to perform measurements on adults as described above, and when it was used to assess children's bones, the following problems arose. These problems will now be described with reference to FIG. 1.
FIG. 1 shows an adult's foot (near the heel). 10 is a calcaneous or heel, 12 is a talus or ankle bone, 14 is a navicular bone, and 16 is a cuboid bone. As the calcaneous 10 comprises a large amount of trabecula bone, structural changes due to bone diseases such as osteoporosis tend to appear often in it, so it is common to diagnose the calcaneous 10 when assessing the bone. Conventionally, a spot 18 (i.e. the irradiating surface area of the ultrasonic beam on the calcaneous) was set to the size of a normal adult's calcaneous as shown in FIG. 1.
However, if the diameter of the spot is not changed and the ultrasonic beam is transmitted to the calcaneous 10 of a child as shown in FIG. 2, a first problem is that the spot 18 of the beam overlaps the calcaneous 10 as shown by the symbol 20. Secondly, another problem is that the ultrasonic beam is transmitted to a join 22 of the bones.
When the first problem occurs, although the object is to perform measurements on the bone, data is obtained also for areas that do not comprise bone, and this adversely affects the reliability of the measurements. Regarding the second problem, the joint between the bones is structurally unique (e.g. the speed of sound is extremely high in that area), so the reliability of bone assessment again falls when the bone is assessed using the speed of sound.
These problems are not limited to ultrasonic measurements on children, and occur also when such measurements are made on adults who have small foot bones.
Another problem was that in the prior art, the measurement point could not be suitably and automatically positioned.
In the bone assessment apparatus disclosed in the aforesaid U.S. Pat. No. 3,847,141, no mechanism to adjust the measurement point was provided, and it was extremely difficult to set this point appropriately according to the size of the subject's foot.
In the aforesaid Tokkai Hei 6-22960, a scanning mechanism is disclosed to position and adjust the measurement point. In this disclosure, the measurement point of the ultrasonic beam is determined based on a two-dimensional X-ray image, but this determination is made by the operator.
In Tokkai Hei 6-327669, a device is proposed for automatically determining the measurement point from an outline image of the bone, however in order to obtain such an outline image, it is necessary for example to scan the bone in two dimensions by an X ray beam.
In the aforesaid Tokkai Hei 7-204205, a mechanism is disclosed to position the measurement point, however no provision is made to control this positioning in accordance with the size of the subject's body part (e.g. foot).
Consequently, in the prior art it was either impossible to adjust the measurement point to suit the size of the body part to be measured, or the adjustment could not be performed without a complex mechanism or an arbitrary human decision.
Bone assessments are performed on many different people, some of whom have large body part (e.g. foot) and of whom others have small body part. Moreover, bone assessments have to be performed not only on adults but also on children. In such cases, when a bone assessment is performed without considering the size of the body part being assessed, the measuring wave (ultrasonic beam or X-ray) may not reach the center of the body part and unexpected reflections or scattering may occur so that the reliability of the results obtained declines.